The synchronous detector, also known as a coherent detector, phase detector, or balanced demodulator, provides a means for detecting synchronous signals in the presence of noise and other interfering signals. The term "synchronous signal" is meant to mean a signal which is synchronous with a reference frequency. The synchronous detector can be configured for extremely narrow bandwidths, which is equivalent to very high Q's, without the sensitive circuits usually required for analog filters at the signal frequency. The synchronous detector is indeed a detector and a filter.
The basic synchronous detector has many implementations. One implementation is shown in FIG. 1. Here the input signal is fed to an inverting amplifier with a gain of one, such that given that the input signal is s(t) the output of the inverter is -s(t). The signal and its inverse are switched by an analog switch driven at the same or synchronous frequency. The analog switch is such that when the digital reference signal is low the inverse is connected to the input of the low-pass filter and when the digital reference signal is high the signal is connected directly to the input of the low-pass filter. The resulting waveform is shown at the output of the switch. For these conditions, the result resembles a fullwave rectified waveform. The switch output waveform is the result of the frequency and phase relationship between the digital reference signal and the input signal. The switch output signal is passed through a low pass filter. The low-pass filter removes the AC components of the waveform giving the average value as a DC voltage at the output. In this case the output is a DC voltage whose magnitude is 0.636 times the peak value of the input wave.
FIG. 2 shows the waveform for various values of the phase angle. In general, if the relative phase of the input signal to the reference is "a" degrees then the output is given by: EQU OUTPUT=0.636.times.PEAK SIGNAL.times.COS(a).
For a quick look at synchronous detection bandwidth, the process of synchronous detection involves multiplying the incoming signal with a squarewave. The result of this product are the sum and difference frequencies of the squarewave and the input signal which includes, for synchronous frequency inputs, a DC term. The low-pass filter, given that the corner is properly set and the input is synchronous, removes all but the DC term.
The corner frequency is defined as that frequency where the signal at the output is attenuated by 3 dB. The filter is said to pass frequencies below the corner and to reject frequencies above the corner. For non-synchronous signals, products below the low pass filter corner are seen in the output. Therefore, the corner of the low-pass filter determines the bandwidth of the detector. The detector rejects all frequencies in the input spectra whose difference from the reference frequency is greater than one half the bandwidth. This can result in some very narrow bandwidths. For example, consider a reference frequency of 100 KHz and a low-pass filter corner of 1 Hz. This results in a detector with a bandwidth of 2 Hz at 100 KHz and something like a Q of 50000. The detector is stable at this Q level because the circuit is not dependent on critically tuned elements. High quality, stable low-pass filters are simple to construct and maintain.
For simplicity, an anti-aliasing prefilter has not been included in FIG. 1. The anti-aliasing filter is used to prevent interference signals whose frequency is near one of the harmonics of the reference frequency from giving an output. Typically the anti-aliasing filter is a low-pass filter with a corner between the reference frequency and its first harmonic. This would be necessary since the synchronous detector also has DC terms for the odd harmonics of the reference signal. The requirements of the prefilter are not severe.
A useful expansion of the synchronous detector circuit shown in FIG. 1 adds another synchronous detector driven by the reference plus 90 degrees. This is shown in FIG. 3. This allows COS (Cosine) and SIN (Sine) outputs whose relation to each other is as follows: EQU COS Output=0.636.times.Peak Signal.times.COS(a), EQU SIN Output=0.636.times.Peak Signal.times.SIN(a).
The synchronous detector has been implemented in many forms and in many products over a long period of time. However, there remains a need for a simple method for detecting and determining the magnitude of either a synchronous frequency signal whose phase relationship to a reference frequency is unknown or a non-synchronous signal near the reference frequency. For a synchronous signal whose phase relationship to the reference signal is unknown or variable there is little meaning to the output of the two detectors illustrated in FIG. 3 without further processing. One method of further processing is to compute the root-mean-square (RMS) of the COS Output and the SIN Output. Dynamically, this method is cumbersome and slow using a computer or inaccurate and tricky using analog multipliers. Thus, what is needed is a fast, accurate system and method which can be employed dynamically and which becomes the primary object of the invention. Other objects will become apparent as the description proceeds.